This invention relates to optical measurements of fine dimensional parameters in integrated circuit (IC) processes and more particularly the measurement of the line width dimensions of a diffraction grating pattern including the thickness of the lines as well as the line edge profile.
In manufacturing integrated circuit (IC) devices and masks many steps are performed in which a pattern of material is disposed on a substrate. The more recent integrated circuits, particularly in the large scale integrated circuit (LSI) art, have tighter lateral dimensional tolerances which require checking at various stages of processing the masks and the substrates.
Many methods are known in the art for measuring the line width of the lines patterned for the IC circuits in various stages of the image transfer process. Line widths can be measured using diffraction gratings and a monochromatic light source such as a laser as described in U.S. Pat. No. 4,200,396, issued on Apr. 29, 1980 to Kleinknecht and Bosenberg and U.S. patent application Ser. No. 101,166, filed Dec. 7, 1979 by Kleinknecht and Bosenberg. According to the procedure described in this patent and application, a diffraction pattern is exposed to a laser beam which generates a plurality of diffracted optical beams. Signals are detected from the beams to generate a signal which is processed to provide information of the line width of the diffraction pattern. This technique is embodied in apparatus disclosed in "Linewidth Measurement on IC Masks and Wafers by Grating Test Patterns" by H. P. Kleinknecht and H. Meier, Applied Optics, 19, 525 (1980), and also described in U.S. patent application Ser. No. 121,529, filed Feb. 14, 1980 by H. P. Kleinknecht and H. Meier, now U.S. Pat. No. 4,330,213, issued May 18, 1982.
In the so-called video disc art, the diffraction-like grooves and the information-generated diffraction-like undulations in the base surface of the grooves are both optically monitored for diffraction signals generated in response to a laser light beamed on the disc. A display is made of the diffraction signals which provides a means to identify defects in the video disc structure. See, by way of example, the following U.S. patents, U.S. Pat. No. 4,180,830, issued Dec. 25, 1979 to W. R. Roach, and U.S. Pat. No. 4,236,823, issued Dec. 2, 1980 to W. R. Roach and P. Sheng. See also an article by W. R. Roach, et al., entitled "Diffraction Spectrometry for VideoDisc Quality Control," R.C.A. Review, Vol. 39, September 1978, p. 472-511.
In the art of making kinescope shadow masks, the aperture size can be measured by exposing the mask to laser light to develop light-diffraction patterns having at least two different interference fringes. Signals representative of the fringes are used to determine the size and shape of apertures in the mask. See Technical Note 1143, published by RCA, and mailed Mar. 16, 1976, entitled "METHOD FOR MEASURING THE AVERAGE APERTURE SIZE OF A PLURALITY OF APERTURES IN A KINESCOPE SHADOW MASK" by A. H. Firester for a description of the method.
In the art of LSI device fabrication there is a need to control the line width, depth and profile of the lines on the IC as it evolves from the original design, through to the final state in which the circuit device is ready for use. This evolutionary manufacturing process is essentially an image transfer process usually implemented by a computerized program to fabricate the basic mask. This process in step-by-step stages causes the desired lines to degrade by varying undesirably in size and uniformity as they are generated in typically silicon and oxide form.